Osteoporosis is a disease characterized by a progressive loss of bone mineral which afflicts millions of post-menopausal women. In post-menopausal women there is an imbalance in bone-forming (osteoblast) and bone-resorbing (osteoclast) cell activities. When bone resorption cell activity exceeds bone formation cell activity, it results in a loss of bone, increased number of bone fractures, incapacitation and increased mortality. Thus reducing the activity of the bone resorptive cell, the osteoclast, would seem to be a useful method for preventing and treating osteoporosis.
It has been observed that the cell surface receptor .alpha..sub.v .beta..sub.3 is the major integrin on osteoclasts responsible for attachment to bone. [Zambonin-Zallone et al., Exp. Cell Res., 182:645-652, (1989); Chambers et al., Bone Miner., 1:127-135, (1986)]. Competitive binding of peptide ligands or MAbs to .alpha..sub.v .beta..sub.3 reduces osteoclast adhesion to matrix proteins and to bone slices and inhibits bone resorption in vitro. [Davies et al., J. Cell. Biol., 109:1817-1826 (1989); Lakkakorpi et al., J. Cell Biol., 115:1179-1186 (1991); Horton et al., J. Bone Min,. Res., 8:239-247 (1993); Horton et al., Exp. Cell Res., 195:368-375 (1991); Helfrich et al., J. Bone Min. Res. 7:335-343 (1992)]. Antagonism of .alpha..sub.v .beta..sub.3 has therefore been associated with predictive activity for decreased bone resorption and a restoration of a more normal balance of bone forming/resorbing activities.
Considering that bone resorption requires a profound, isolated extracellular pH gradient at the osteoclast-matrix interface [Vaes, Clin. Orthop., 231:239-271 (1988)], effective cell anchoring is a pivotal event. The attachment of cells to matrix is mediated by highly conserved membrane proteins known as integrins [Yamada, J. Biol. Chem., 266:12809-12812 (1991)]. These are non-covalently-bound heterodimers, each consisting of an .alpha. and .beta. subunit which associate in limited combinations [Albelda et al., FASEB J., 4:2868-2880 (1990)]. .alpha..sub.v .beta..sub.3 is the major integrin on osteoclasts responsible for attachment to bone [Zambonin-Zallone et al., Exp. Cell Res., 182:645-652 (1989); Chambers et al., Bone Miner., 1:127-135 (1986)]. A monoclonal antibody which inhibits the resorptive activity of osteoclasts [Chambers et al., Bone Miner., 1:127-135 (1986)] recognizes .alpha..sub.v .beta..sub.3 in osteoclastoma tissue [Davies et al., J. Cell Biol., 109:1817-1826 (1989). .alpha..sub.v .beta..sub.3 recognizes RGD sequences [Smith et al., J. Biol. Chem., 265:2168-2176 (1990)] in a variety of matrix macromolecules, including bone sialoprotein and osteopontin [Ross et al., J. Biol. Chem., 268:9901-9907 (1993); Oldberg et al., J. Biol. Chem., 263:19433-19436 (1988); Reinholt et al., Proc. Natl. Acad. Sci. USA, 87:4473-4475 (1990)], two proteins found in bone. Several studies indicate that the RGD-containing peptides, including the snake venom peptide echistatin, are potent inhibitors of osteoclastic activity both in vitro [Sato et al., J. Cell Biol., 111:1713-1723 (1990)] and in vivo [Fisher et al., Endocrinology, 132:1411-1413 (1993)]. However, it should be noted that RGD peptides in general are non-selective for RGD-dependent integrins. For example, most RGD peptides also bind to .alpha..sub.v .beta..sub.5 and .alpha..sub.v .beta..sub.1.
European Patent Application 496,378 discloses amidinobiphenyl compounds which inhibit cell-cell and cell-matrix interaction and are useful for treating thrombosis, cerebrovascular diseases, pulmonary embolisms, myocardial infarction, arteriosclerosis, osteoporosis and tumour metastases.
WO 93/09795 discloses non-peptide RGD analogs having terminal guanidino and carboxyl functions spaced by a chain of 11 atoms, at least 5 of which are carbon atoms, and containing no sequence of .alpha.-amino acids. These compounds inhibit platelet aggregation and are useful for the treatment of several pathological disorders.
Vitamin D, calcium and thiazide diuretics have been used alone or in combination to prevent bone loss associated with corticosteroids. The goal of such therapy is to improve calcium absorption and decrease urinary excretion of calcium thus reversing secondary hyperparathyroidism [J. C. Joseph, Am. J. Hosp. Pharm., Vol. 51, pp 188-197 (1994)]. Calcium supplements are widely used in managing established osteoporosis but there have been few satisfactory prospective studies of calcium supplementation on bone density or the risk of further fracture [Cooper et al., Quarterly Journal of Medicine, 87:203-209 (1994)]. Estrogen replacement therapy has been investigated as a means of preventing perimenopausal bone loss [Cooper et al., Quarterly Journal of Medicine, 87:203-209 (1994)] but has been met with poor patient compliance due to the fear of increased risk of breast and uterine cancer, weight gain and continued menses. The bisphosphonates, etidronate, tiludronate, clodronate, alendronate and residronate and calcitonin have been explored for use as antiresorptive drugs but also are associated with significant concerns or side effects. Thus it would be beneficial to provide antagonists of osteoclast .alpha..sub.v .beta..sub.3 which are effective inhibitors of bone resorption in vivo which will have a competitive advantage for the treatment of human postmenopausal osteoporosis.